One conventional optical head comprises a semiconductor laser from which a laser beam radiates, a collimating lens for collimating the laser beam, a polarizing beam splitter for passing the laser beam from the semiconductor laser and for reflecting the laser beam which is reflected from an optical disk, a 1/4 wavelength plate for converting the laser beam which is passed through the polarizing beam splitter into a circularly polarized laser beam and for converting the laser beam which is reflected from the optical disk into a linearly polarized laser beam having a polarized plane orthogonal to a polarization of the laser beam path, a total reflecting prism for reflecting the laser beam which is passed through the 1/4 wavelength plate and which is reflected from the optical disk, an object lens for focusing the laser beam from the semiconductor laser on the optical disk and for collimating the laser beam which is reflected from the optical disk, a focusing lens for focusing the laser beam which is reflected by the polarizing beam splitter, a beam splitter for dividing the laser beam which is focussed by the focusing lens into passing and reflecting beams, a first two-divided optical detector for receiving the laser beam which is passed through the beam splitter to provide a tracking error signal and RF signal, a knife-edge for shielding a half of the laser beam which is reflected by the beam splitter, and a second two-divided optical detector for receiving a remaining half of the laser beam which is not shielded by the knife-edge to provide a focus error signal.
In operation, the laser beam which is radiated from the semiconductor laser is collimated by the collimating lens, and then passed through the polarizing beam splitter. The laser beam which is passed through the polarizing beam splitter is converted into the circularly polarized beam, and then reflected by the total reflecting prism. Thereafter, the laser beam is focussed on the optical disk by the object lens, and then reflected therefrom to be reflected again by the total reflecting prism. The laser beam thus reflected is converted into the linearly polarized laser beam having a polarized plane orthogonal to the polarization of the laser beam path by the 1/4 wavelength plate, and then reflected by the polarizing beam splitter. Thereafter, the laser beam is focussed by the focusing lens. The laser beam thus focussed is partially passed through the beam splitter to be received by the first two-divided optical detector from which the tracking error signal is obtained in the calculation of the subtraction between signals of two optical detecting elements therein by the push-pull method, and the RF signal is obtained in the calculation of the addition between the signals thereof. On the other hand, a half of the laser beam which is reflected by the beam splitter is not shielded by the knife edge to be received by the second two-divided optical detector from which the focus error signal is obtained in the calculation of the subtraction between signals of two optical detecting elements therein by the Foucault method.
According to the conventional optical head, however, there is a disadvantage that an optical head which is put into a practical use is larger in size than, for instance, 40.times.40.times.30 mm.sup.3 so that it is bulky and heavy. There is a further disadvantage that light beams supplied to the two optical detecting elements become imbalanced because the light axis of the object lens is deviated in regard to the dividing line of the first two divided optical detector when the object lens is moved in a direction vertical to the light axis by an actuator which is operated in accordance with a tracking error signal. Consequently, there results a still further disadvantage that a direct current offset is induced in the tracking error, signal so that a scope becomes narrower in controlling the tracking error. In addition, there is a yet further disadvantage that a manufacturing cost is high because many parts which are necessary to be polished optically are used therein.